1. Field of the Invention
This invention relates to an optical recording medium having tracks capable of recording optically reproducible data, and more particularly to an optical recording medium having tracking tracks for following tracks capable of recording data, that is, for tracking, and also to a process for producing the optical recording medium.
2. Related Background Art
Generally, an optical recording medium, such as an optical disk, an optomagnetic disk, etc. has data recording tracks (which will be hereinafter referred to as "recording tracks") and tracking tracks formed in a manner optically discriminatable from the recording tracks so that a recording-reproducing beam may correctly follow the recording tracks. On the recording tracks which are each interposed between the tracking tracks, address data for providing positional information on the recording tracks to an optical head control system, etc. are formed in advance as prepits.
The prepits are recesses each having a depth of an odd number multiple of .lambda./4n (wherein .lambda. is the wavelength of reproducing beam and n is the refractive index of the substrate) so selected as to usually provide a maximum modulation to the reproducing beam, that is, a tracking beam, irradiating the optical recording medium so that the reproducing beam may pass through the substrate. As tracking tracks, grooves formed on the substrate are usually used.
So far known methods for detecting tracking errors of such an optical disk include the so called push-pull method and the three-beam method. The push-pull method comprises using a single beam and outputting the difference in the outputs of reflected and diffracted beam lights of an incident single beam on and from the disk at two light-receiving sections of two divided parts of a photodiode, the two divided parts being symmetrically provided at the track center, thereby detecting a tracking error. The three-beam method comprises using a main beam for recording and reproducing data onto and from recording tracks and further using primary light diffracted by a diffraction grating as auxiliary beams at the same time, and outputting the difference in the outputs of the reflected light of the auxiliary beams at two light-receiving sections receiving the reflected light, thereby detecting a tracking error.
When the prepit length and the prepit distance are shortened in such an optical recording medium to increase the recording density, as shown, for example, in FIGS. 15 and 16, the amplitude W.sub.16 of quantity of reflected light (difference in the brightness) is decreased and the S/N of the prepits is also lowered, if the spot size of reading beam is constant. This tendency is particularly remarkable for prepits formed on the recording tracks on the inner peripheral portion of a disk-type optical recording medium, because, when a reading light spot is placed between one prepit and another, no sufficient quantity of reflected light can be obtained due to the interference from the preceding and successive prepits. If the pit size of the prepits is reduced to decrease the interference from the prepits, insufficient darkness can be obtained when the reading light spot is on the prepit, and thus there is no more improvement in the signal output. That is, even if the pit shape is changed, there are still such problems that in case the reading light spot size is constant, no sufficient signal output can be obtained at parts of the disk having a shorter pit distance when the recording density of address data is increased.